The present invention relates to a ball attaching apparatus and a method of attaching solder balls using the same, and more particularly to a ball attaching apparatus which can precisely attach solder balls to respective ball lands of a substrate even though warpage occurs in the substrate, and a method for attaching solder balls using the same.
Two key areas of semiconductor package development are miniaturization and improved electrical characteristics. A ball grid array (BGA) package is one such semiconductor package generally regarded as being capable of meeting the requirements.
The size of the entire BGA package is about the same or comparable to that of a semiconductor chip such that the mounting area of a BGA package can be minimized. A BGA package is electrically connected to the external circuits by solder balls, and this leads to improved electrical characteristics as the electric signal transmission paths of the solder balls are short.
The semiconductor chips of recent design are characterized with more miniaturization and higher functionality with more signal input and output pads. To meet this requirement, the BGA packages are manufactured as FBGA (fine pitch BGA) packages.
FIG. 1 shows a cross-sectional view of a conventional FBGA package.
Referring to FIG. 1, a semiconductor chip 100 is attached by an adhesive 109 to a substrate 102, which has a cavity 107 at a center portion thereof. The bonding pads 101 of the semiconductor chip 100 are electrically connected with the electrode terminals 103 of the substrate 102 by metal wires 106, which pass through the cavity 107. The upper surface of the substrate 102 including the semiconductor chip 100, the cavity 107 including the metal wires 106 in the cavity 107 are molded using a molding material 108. Solder balls 105 for mounting the FBGA package of FIG. 1 to external circuits are attached to the ball lands 104 provided on the lower surface of the substrate 102.
The solder ball attaching process is implemented using a ball attaching apparatus by vacuum-sucking the molten solder through the eject pins of the ball attaching apparatus, positioning the eject pins on the ball lands 104, and then dropping solder balls 105 on the ball lands 104 by removing the vacuum in the eject pins.
On the other hand, the substrate warpage could occur in the course of assembling a semiconductor package. Any warped substrate would cause irregular variations in the intervals between the balls lands leading to misalignment of the eject pins for dropping the solder balls and the ball lands to which the solder balls are to be dropped, as such solder balls cannot be precisely attached onto the ball lands formed on a warped substrate.
FIG. 2 shows the types of warpage occurring in a substrate, and FIG. 3 is shows the problematic attachment patterns of solder balls due to warpage of a substrate.
Referring to FIG. 2, the substrates could be warped during assembly of a semiconductor package into different shapes of a “smiling” type, a “crying” type, a “gull” type, and an “inversed gull” type. These types of warpage produce intolerant variations in the gaps between the ball attaching apparatus and the ball lands provided on the warped substrate such that a uniform distance between ejector pins and the balls lands cannot be maintained. The gaps between the ball attaching apparatus and the ball lands provided on the warped substrate could vary from a small value less than 300 μm to a large value greater than 900 μm.
Accordingly, due to misalignment of the ball attaching apparatus and the ball lands on a warped substrate, a solder ball could miss a ball land or an extra solder ball could be attached to a ball land or a solder ball could be miss-positioned on a ball land due to the fact that the solder balls are not precisely dropped onto the ball lands formed on a warped substrate during a solder ball attaching process.
For example, now referring to FIG. 3, the solder balls used in a BGA package may have a height of 400˜450 μm, and the warpage of a substrate occurs in the smiling type. In this example, the gap between a set of eject pins and the ball lands near an edge of the warped substrate could be less than 300 μm, as such it would be impossible to secure the sufficient gap required for proper attachment of the solder balls.
As the solder balls are pressed by eject pins during a ball attaching process (due to the insufficient gaps as explained above), the eject pins may not drop the solder balls on the intended ball lands but instead carry away the solder balls to a different location causing the “missed ball” phenomenon and the “extra ball” phenomenon as shown in FIG. 3.
Toward the center portion of the substrate warped in a “smiling” type, the gap between eject pins and the ball lands may be greater than 900 μm, which is at least two times larger than the height of solder balls. This will lead to the “mis-positioned ball” phenomenon (see FIG. 3), in which the solder balls are amispositioned on the ball lands when attaching the solder balls.
The variations in the gap between the ball attaching apparatus and the ball lands on a warped substrate cannot ensure proper attachment of solder balls on the ball lands such that it will cause defects and lower the productivity yield.